A vehicle-equipped nonaqueous electrolyte secondary battery used in, e.g., a power supply for driving an electric vehicle (EV) or a hybrid electric vehicle (HEV or PHEV) includes a pressure-sensitive current interrupt mechanism in addition to a safety valve for the prevention of explosion. The pressure-sensitive current interrupt mechanism is operated with gas that is quickly generated inside the battery in an abnormal state. The current interrupt mechanism is provided to interrupt a current that is going to flow toward the outside, and to prevent a burst of the battery or firing.
In the nonaqueous electrolyte secondary battery, raising a charging voltage is known as one of methods for increasing the battery capacity. As a safety measure in consideration of the event of an overcharge state of the nonaqueous electrolyte secondary battery, it is also known to add, into a nonaqueous electrolytic solution, one or more overcharge inhibitors such as tert-amylbenzene and/or biphenyl (see Patent Literature (PTL) 1), or a cycloalkyl benzene compound and/or a compound having quarternary carbon adjacent to a benzene ring (see PTL 2). However, raising the charging voltage to increase the battery capacity accompanies with a risk that the overcharge inhibitor may be decomposed even at a voltage, which is set within the range for ordinary use, depending on the type of the overcharge inhibitor, and that battery characteristics and safety may degrade after charge and discharge cycles.
From the viewpoint of solving the above-mentioned problem, it is further known to improve overcharge tolerance by adding lithium carbonate (Li2CO3) to a positive electrode mixture of the nonaqueous electrolyte secondary battery (see PTL 3). When lithium carbonate is added to the positive electrode mixture of the nonaqueous electrolyte secondary battery, carbon dioxide is generated from a positive electrode plate upon application of a high voltage to the battery, e.g., in the event of overcharge. As a result, the pressure-sensitive current interrupt mechanism can be caused to operate reliably with the generated carbon dioxide at earlier timing than the operation of the safety valve for the prevention of explosion.